Tumor promotion phenomena in two-stage carcinogenesis were systematically explored in various rodent species in conjunction with transplacental carcinogenesis. Structure-promoting activity relationships of various barbiturates, hydantoins and benzodiazepine tranquilizers were investigated by sequential administration to animals of a transient, low level exposure to a genotoxic carcinogen followed by the test agent under study. Two long-acting hypnotic barbiturates, allobarbital and aprobarbital, and one intermediate-acting compound, pentobarbital, were found to promote liver carcinogenesis in male rats, while two monosubstituted nonhypnotic barbiturates and an intermediate-acting barbiturate, secobarbital, lacked such activity. A long-acting sedative anticonvulsive agent, nirvanol (5-ethyl-5-phenylhydantoin), promoted the development of hepatocellular tumors while a nonhypnotic hydantoin, 5,5- diethylhydantoin, was ineffective. A close relationship was found to exist between the induction of certain cytochrome P- 450 species and tumor promoting abilities of barbiturates and hydantoins. Unlike the rat and mouse, in the Syrian golden hamster liver parenchymal cells were resistant to tumor promotion by phenobarbital. Phenobarbital increased liver weight and enhanced hepatic alkoxyresorufin O-dealkylase and aminopyrine N-demethylase activities in rats and mice susceptible to liver tumors but failed to induce any of these parameters in hamster liver to a significant extent.